1. Field of Invention
The field of this invention is refrigeration. More specifically, the present invention relates to an improved system and method of defrosting low temperature evaporators. As an example the typical retail supermarket or grocery store has many low temperature cases for displaying frozen or refrigerated food. The refrigeration system typically uses a multiplicity of evaporators, a plurality of compressors and one or more condensers. As the evaporators operate at temperatures well below the frost point of water vapor, the evaporators become covered with frost during operation. The frost will build up until the evaporator can no longer function efficiently, so it must be periodically taken out of service and defrosted. The present invention relates to a system and method of using the latent heat of high pressure saturated refrigerant gas to defrost the evaporators sequentially while returning the liquid and gaseous refrigerant to the suction side of the compressor(s) in a more efficient manner than previously possible.
2. Description of Prior Art
There are four basic methods of defrosting evaporators. The first is to simply turn off the evaporator to be defrosted. This method is very slow, only works when the ambient temperature in the area around the evaporator is above 28.degree. F. and may affect the room temperature.
The second method of defrost is to use electric heat. This method is simple but is not an efficient use of energy. The typical way in which electric defrost is accomplished is to put an electric heating element between the evaporator fan and the coil so that warm air is circulated through the coil in order to melt the buildup of frost.
The third method of defrost is to use store air to melt the frost. This method causes ambient air in the store to be blown through the evaporator to accomplish defrost.
The fourth method of defrost is properly called latent heat defrost but is generally known as hot gas defrost. This method uses a change of state in refrigerant, from a saturated gas to a liquid, to heat the evaporator coil from the inside causing the frost to melt. This method, as well as the other three, are all generally known to those skilled in the refrigeration art.
Most commercial hot gas defrost systems presently utilize a pressure reducing valve placed in the high pressure liquid supply conduit so that when an evaporator goes into defrost the liquid being forced out of the evaporator coil is forced into a liquid supply header. A typical system, known commonly as a two pipe system, is disclosed in Ares et al. U.S. Pat. No. 4,621,505. Also commonly known is a three pipe system, generally this type of system is not used due to its higher cost of installation.
One of the problems with the two pipe hot gas defrost system is what to do with the liquid refrigerant in the evaporator and its associated liquid supply line during the defrost cycle. Not only does the liquid refrigerant in the evaporator coil move into the liquid supply conduit but any refrigerant condensed during the defrost cycle, as well as some hot gas, moves into the liquid supply conduit. This can increase the temperature of the supply liquid, causing so-called flash gas as well as introducing hot gas directly into the liquid supply lines of the evaporators not being defrosted. The major problem with this method of defrost is that any time there is a reduction in pressure of the liquid refrigerant, without a corresponding reduction in temperature, the propensity for flash gas to occur is aggravated. In order to avoid flash gas in the liquid supply lines the liquid and gaseous refrigerant outflow from the hot gas defrost should be returned to the refrigeration circuit at a point other than the liquid supply header. The problem of flash gas, especially in conjunction with a hot gas defrost means that utilizes a pressure reducing valve in the liquid supply line, is well known to the art. The nexus of the problem is in how to return the liquid and gaseous refrigerant products of hot gas defrosting back into the normal refrigeration circuit in an efficient manner without disrupting the high and low pressures of the evaporators still operating in the normal refrigeration mode.
Quick U.S. Pat. No. 3,234,754 discloses a hot gas refrigeration defrost system which addresses the problem of utilizing hot gas refrigerant to defrost a number of separate evaporator coils and then return the defrosting refrigerant to the refrigeration circuit as a gas. The system disclosed by Quick, noting particularly FIG. 2 and the accompanying description of the patent, involves use of a heat exchanger, called an intercooler, to evaporate by heat exchange the refrigerant used for defrosting with the heat of vaporization being supplied by the high pressure liquid refrigerant used elsewhere in the system for evaporator cooling. Conversion of the defrosting refrigerant from liquid state to gaseous state occurs in the intercooler in part by such heat exchange and in part by use of an aspirator which introduces a "small quantity" of liquid refrigerant into the gaseous refrigerant being drawn from the intercooler and delivered to the suction side of the compressors. As is well known, and acknowledged by Quick, any liquid introduced to the compressor(s) of the system is "highly undesirable" and it must be said of the Quick defrosting refrigerant reintroduction technique that it is risky in this respect, as a practical matter.
The present invention, however, effectively accomplishes the purpose (reintroduction of the defrosting refrigerant to the system) without risk to the compressor(s) by use of an expansion valve (to reduce the pressure without causing a phase change) on the defrosting refrigerant flowing into the heat exchanger; with the combined use of both the expansion valve and the heat exchanger further ensuring evaporation of the liquid refrigerant, and then delivery of the refrigerant gas to the compressor(s) intake.
Analyzing the Quick defrost system in another respect, in the Quick system, as shown in FIG. 2, the hot gas used to defrost is taken directly from the compressor 110 outlet which results in a high pressure of refrigerant through the evaporator being defrosted, with only slight reduction of pressure in the evaporator. Quick accumulates the refrigerant from the defrosting evaporator in an intercooler 193 with the outtake from the intercooler being in gaseous state only by action of aspirator 197 leading to conduit 196 which in turn delivers the refrigerant to the suction side of compressor 110. Since the intercooler 193 is at high pressure and receives refrigerant in both liquid and gaseous form, and since the only outtake is in gaseous form, the liquid refrigerant will accumulate in the intercooler 193 at least until the aspirator 197 starts introducing some liquid to the compressor(s).
Alternatively, the pressure in intercooler 193 is uncontrolled because aspirator 197 does not control pressure; hence, the pressure increases both in the intercooler 193 and at the suction side of compressor 110 to the point where the compressor efficiency is lost or the compressor overheats due to the high pressure in the suction line.
Another problem with the Quick system that is solved by the present invention is how to return compressor crankcase oil to the compressors. Even with oil separators a sufficient amount of oil is pumped through the refrigeration system that there must be provision for its return. In the Quick system, the oil will collect in the intercooler since there is no means to either evaporate the oil or to return the oil as a liquid. In the present invention, the expansion valve in the return line allows the oil entrapped in the refrigerant to be condensed into a liquid in the subcooler and then returned to the compressor crankcase as a liquid by using a standard P-trap arrangement (not shown).
Quick U.S. Pat. No. 3,645,109 discloses a means for dealing with flash gas by using a chamber in the liquid supply circuit to separate the flash gas from the liquid refrigerant, the flash gas being created by a pressure reducing valve in the liquid supply line and from defrost cycle. In one embodiment, FIG. 5, Quick uses a solenoid valve on the main liquid refrigerant supply line in order to change the pressure in the liquid supply line in order to return the liquid refrigerant collected in the flash gas, liquid refrigerant separation chamber back into the liquid supply line. The present invention does not return any liquid to the liquid supply line, nor is the liquid supply line to all of the system evaporators ever interrupted.